Characterization of display devices by averaging chromaticity values

ABSTRACT

The present invention provides a method for characterizing display devices. Initially, a plurality of colors are generated on the display device. The generated colors are measured and a black point and a white point are determined. The measured colors are then corrected for the determined black point in order to obtain a plurality of chromaticity values. The chromaticity values of the corrected color values are averaged, and a tristimulus matrix is generated with the averaged chromaticity values and the determined white point. By averaging the chromaticity values of black-point-corrected measurements, the present invention is able to create more accurate display device characterizations that account for the effects of flare. In addition, by averaging the chromaticity values of black-point-corrected measurements, the present invention minimizes the effects of inaccurate color measurements made during the device characterization process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the generation of color models whichcharacterize display devices.

2. Description of the Related Art

Typically, display devices, such as liquid crystal displays (LCD) orcathode-ray tubes (CRT), produce colors on a screen by combiningdifferent amounts of red, blue and green light. The actual colorproduced from specific combinations of RGB light varies from displaydevice to display device. Therefore, in order to consistently reproducecolors on different displays, manufacturers of display devices createcolor models that characterize the output of a particular display devicein terms of a standard color coordinate system. For instance, CommissionInternationale de l'Enclairage (CIE) XYZ color coordinates are oftenused.

Conventionally, a display device characterization is represented by thefollowing equation: $\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\lbrack M\rbrack \cdot \begin{bmatrix}R^{\prime} \\G^{\prime} \\B^{\prime}\end{bmatrix}}$

R′G′B′ are radiometric scalar values that are related to device RGBvalues according to generally non-linear relationships commonly called“gamma curves.” The matrix M is a 3×3 tristimulus matrix which relatesthe linear R′G′B′ values to XYZ values.

Conventionally, the tristimulus matrix M is populated according to thefollowing equation: $M = \begin{bmatrix}X_{r,\max} & X_{g,\max} & X_{b,\max} \\Y_{r,\max} & Y_{g,\max} & Y_{b,\max} \\Z_{r,\max} & Z_{g,\max} & Z_{b,\max}\end{bmatrix}$Each X, Y, and Z value in matrix M corresponds to a measured XYZ valuewhen the maximum R, G and B values are applied to the device. Oncedetermined, the tristimulus matrix M maps any linearized R′G′B′ valueused by the display device to the actual XYZ color that the displaydevice is expected to reproduce.

This conventional model for characterizing display devices is built uponthe assumption that the measurements are done in a dim environment whereambient lighting has little effect on the measurements. This assumptionmay not always prove true. For example, many display devices willproduce a non-zero amount of light when displaying the color black. Thisblack point measurement is often called flare. In addition, a non-zeroflare value can be caused by higher than optimal ambient lighting. Ineither case, the flare skews the results of the measurements.

Attempts to measure or estimate flare, and correct for its effect, arewell-known in the art. However, these attempts have generally beenunsatisfactory. For instance, the effect of flare has been built into ageneral gain-offset-gamma-offset (GOGO) model for gamma curves. Thismodel attempts to correct for the effects of flare in gamma curves.However, the effect of this model is limited since each gamma curve canonly act on each channel independently. Other efforts have been made tocorrect for flare in the tristimulus matrix, but these efforts stillutilize the conventional formula for populating the matrix.

As such, the conventional method does not fully address problemsassociated with flare, since only measurements at maximum RGB values areused to populate the tristimulus matrix. Even if flare is accounted for,the measurements themselves may be inaccurate, and therefore, theresulting tristimulus matrix may not provide the most accurate devicecharacterization.

SUMMARY OF THE INVENTION

The present invention addresses the foregoing problems by obtainingvalues for a tristimulus matrix by averaging the chromaticities ofmeasured values that have been corrected by the black point.

According to one aspect of the invention, a plurality of colors aregenerated on the display device. The generated colors are measured and ablack point and a white point are determined. The measured colors arethen corrected for the determined black point in order to obtain aplurality of chromaticity values. The chromaticity values of thecorrected color values are averaged, and a tristimulus matrix isgenerated with the averaged chromaticity values and the determined whitepoint.

According to more preferred aspects of the invention, the plurality ofcolors are generated by the display device in accordance with aplurality of digital values in red, blue, green and neutral ramps.

According to another aspect of the invention, a plurality of gammacurves are created by inverting the tristimulus matrix and multiplyingthe inverted tristimulus matrix with the measured colors from theneutral ramp.

According to yet another aspect of the invention, the black point isdetermined by measuring the color produced at the neutral ramp value ofR=G=B=0.

According to still another aspect of the invention, the black point isdetermined by estimating the black point from the plurality of measuredcolors using a non-linear optimization method.

By averaging the chromaticity values of black-point-correctedmeasurements, the present invention is able to create more accuratedisplay device characterizations that account for the effects of flare.In addition, by averaging the chromaticity values ofblack-point-corrected measurements, the present invention minimizes theeffects of inaccurate color measurements made during the devicecharacterization process.

This brief summary has been provided so that the nature of the inventionmay be understood quickly. A more complete understanding of theinvention can be obtained by reference to the following detaileddescription of the preferred embodiment thereof in connection with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative view of the environment in which the presentinvention may be implemented according to one embodiment of theinvention.

FIG. 2 is a graph depicting the measured values of the R, G and B rampsin the x-y chromaticity plane.

FIG. 3 is a flow chart for explaining the process to create a displaydevice characterization color model according to one embodiment of thepresent invention.

FIG. 4 is a graph depicting the measured values of the R, G and B rampsin the x-y chromaticity plane that have been corrected for theblack-point.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for generating a color modelthat characterizes display devices by utilizing averaged, black-pointcorrected, chromaticity values from measured RGB values to populate atristimulus matrix. Typically, the present invention is implemented in acomputing environment. A representative computing system may includecomputing equipment, peripherals and digital devices which may be usedin connection with practicing the present invention. The computingequipment includes a host processor which comprises a personal computer(hereinafter “PC”), preferably an IBM PC-compatible computer having awindowing environment such as Microsoft Windows 98, Windows 2000,Windows Me, Windows XP, or Windows NT, or other windowing systems suchas LINUX. In the alternative, the host processor may be an Applecomputer or other non-windows based computers. The computing equipmentalso includes a color monitor, including a display screen, a keyboardfor entering text data and user commands, and a pointing device. Thepointing device preferably comprises a mouse for pointing and formanipulating objects displayed on the display screen.

The computing equipment also includes computer-readable memory mediasuch as a computer fixed disk and a floppy disk drive. A floppy diskdrive provides a means whereby the computing equipment can accessinformation, such as color measurement data, computer-executable processsteps, application programs, etc. stored on removable memory media. Inthe alternative, information can also be retrieved through other meanssuch as a USB storage device connected to a USB port, or through anetwork interface. Also, a CD-ROM drive and/or a DVD drive may beincluded so that the computing equipment can access information storedon removable CD-ROM and DVD media.

The internal architecture of the host processor of the computingequipment includes a central processing unit (CPU) which interfaces witha computer bus. Also interfacing with computer bus are a fixed disk, anetwork interface, a random access memory (RAM) for use as a mainrun-time transient memory, a read only memory (ROM), a floppy diskinterface, a display interface for the monitor, a keyboard interface forthe keyboard, a mouse interface for the pointing device, and aninterface for other peripheral digital devices, such as a colorimeter ora spectroradiometer.

The RAM interfaces with the computer bus so as to provide informationstored in the RAM to the CPU during execution of software programs suchas an operating system, application programs, such as a display devicecharacterization program, and device drivers. More specifically, the CPUfirst loads computer-executable process steps from the fixed disk, oranother storage device, into a region of the RAM. The CPU can thenexecute the stored process steps from the RAM in order to execute theloaded computer-executable process steps. Data such as colormeasurements or other information can be stored in the RAM, so that thedata can be accessed by CPU during the execution of computer-executableprocess steps need to access and/or modify the data.

The fixed disk contains the operating system, and application programs,such as a display device characterization program. The display devicecharacterization method of the present invention is preferablyperformed, in part, by computer-executable process steps which arestored on the fixed disk for execution by the CPU, such as in one of theapplication programs. The process steps for characterizing a display ofthe present invention are described in more detail below.

FIG. 1 depicts a representative environment in which the invention ispracticed. In order to characterize the output of an RGB display device,many different RGB colors must be displayed on a display device andmeasured. As shown in FIG. 1, computing equipment 100 is used to supplyRGB values to display device 101. Computing equipment 100 controlsdisplay device 101 to display a supplied RGB value as color patch 103.Color measuring device 102 is used to measure the color of color patch103. Color measuring device 102 can be a spectroradiometer or acolorimeter, or any other color measuring device that is capable ofmeasuring colors and outputting the measurements in the CIEXYZ format.

Computing equipment 100 controls display device 101 to display colors inred, blue, green and neutral ramps on color patch 103. For example, inthe preferred embodiment, the red ramp consists of 17 R values rangingfrom digital values of 15 to 255 in steps of 15. While using 17 valuesin the ramp is preferred, more or less values of R could be used. The Gand B values are held at zero for each R value in the red ramp. Thegreen and blue ramps are constructed in the same manner as the red ramp.

Color measuring device 102 takes a measurement of color patch 103 foreach value in each ramp, and supplies the measurements to computingequipment 100. For example, color measuring device 102 can be connectedto computing equipment 100 via a USB cable or another device interface.FIG. 2 depicts a graph showing representative measured values of thered, green, and blue ramps on the chromaticity x-y plane. Dotted line201 represents the measured chromaticities of the green ramp, dottedline 202 represents the measured chromaticities of the red ramp, anddotted line 203 represents the measured chromaticities of the blue ramp.The chromaticities of each ramp converge as the digital values of eachR, G, and B ramp approaches zero.

In addition to displaying and measuring colors along each red, blue andgreen color ramp, the present invention also displays and measurescolors along a neutral ramp. The neutral ramp contains digital valueswhere R=G=B. For a cathode-ray tube (CRT) display device, the neutralramp is preferably measured at digital values of R=G=B=0, 8, 16, 32, 64,128, 192 and 255. For a liquid crystal display (LCD) device, the neutralramp is preferably measured at digital values of R=G=B ranging from 0 to255 in steps of 15. In either case, the digital value of R=G=B=0 causesthe color black to be displayed on color patch 103, and R=G=B=255 causesthe color white to be displayed on color patch 103.

Color measuring device 102 takes a measurement of color patch 103 foreach value in the neutral ramp, and supplies the measurements tocomputing equipment 100. The measurement of the color black is oftenreferred to as the “black point” of the display device, and likewise,the measurement of the color white is referred to as the “white point”of the display device.

Once all the measurement have been taken and supplied to computingequipment 101, a characterization for display device 102 can becomputed. A display device characterization is typically represented bythe following model: $\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\lbrack M\rbrack \cdot \begin{bmatrix}R^{\prime} \\G^{\prime} \\B^{\prime}\end{bmatrix}}$R′G′B′ are radiometric scalar values that are related to device RGBvalues according to generally non-linear relationships commonly called“gamma curves.” The matrix M is a 3×3 tristimulus matrix which relatesthe linear R′G′B′ values to XYZ values. This model can be utilized by acolor management module (CMM) contained in a device driver or computeroperating system to convert device-independent XYZ color values into theRGB values used by the characterized display device.

FIG. 3 depicts a flow chart that shows how the present inventionpopulates the tristimulus matrix M. Initially, in step S 301, computingequipment 100 obtains the measured values of each value in each colorramp from the color measuring device 102. In step S 302, the black pointof display device 101 is determined. It is necessary to determine theblack point (also known as flare) so that its effects may be correctedfrom the other measured data points. Ideally, when a display devicedisplays the color black, the measured output of the display deviceshould read (0,0,0) in XYZ. However, many display devices will produce anon-zero amount of light when displaying the color black. In addition, anon-zero black point may be due to higher than optimal ambient lightingwhen the color measurement is taken. In either case, the flare skews theresults of the measurements.

The black point can be determined in multiple ways. First, the blackpoint can simply be measured from the display device. As stated above,the neutral ramp value of R=G=B=0 produces the color black, and itsmeasured value is the black point. However, since color measuringdevices are typically inaccurate at low light levels, using the directlymeasured black point of a display device is not typically an accuratemethod determining the black point.

Instead, it is preferable to obtain the black point of the displaydevice by estimating its value from more accurate color measurements.The estimation process involves solving a nonlinear optimization problemto find the “best fit” black point that minimizes the drift inchromaticity along the R. G, and B channels. The nonlinear optimizationproblem solves for the black point (X_(K), Y_(K), Z_(K)) that minimizesthe following objective function:Φ(X _(K) , Y _(K) , Z _(K))=Ψ(X _(K) , Y _(K) , Z _(K) ; S _(R))+(X _(K), Y _(K) , Z _(K) ; S _(G))+(X _(K) , Y _(K) , Z _(K) ; S _(B))

S_(R), S_(G), and S_(B) are the set of data points corresponding to themeasured values of the R, G, and B ramps described above. For any set ofS, the function Ψ is defined as follows.${\Psi\left( {X_{K},Y_{K},{Z_{K};S}} \right)} = {\sum\limits_{i \in S}{{{f\left( {X_{i},{Y_{i};X_{K}},Y_{K},Z_{K}} \right)} - {\overset{\_}{f}\left( {X_{K},Y_{K},{Z_{K};S}} \right)}}}^{2}}$${f\left( {X,Y,{Z;X_{K}},Y_{K},Z_{K}} \right)} = \left( {\frac{X - X_{K}}{X - X_{K} + Y - Y_{K} + Z - Z_{K}},\frac{Y - Y_{K}}{X - X_{K} + Y - Y_{K} + Z - Z_{K}}} \right)$${\overset{\_}{f}\left( {X_{K},Y_{K},{Z_{K};S}} \right)} = {\frac{1}{S}{\sum\limits_{i \in S}{f\left( {X_{i},Y_{i},{Z_{i};X_{K}},Y_{K},Z_{K}} \right)}}}$

In the above equations, |S| is the cardinality of S, i.e., the number ofpoints in the set S. The function f gives the chromaticity coordinatesof the point (X−X_(K), Y−Y_(K), Z−Z_(K)), and the function f_(bar) givesthe average, or center of mass, of all the points in the set S in thechromaticity plane. Thus, the function Ψ gives the sum of second momentsof the points about the center of mass, and is a measure of how spreadout the points are about the center of mass. The function Φ is a totalmeasure of how spread out the three clusters of points from the measuredRGB color ramps are about their respective centers of mass.

Just as the direct measurement of the: black point is often inaccuratedue to the insensitivity of color measuring devices at low light levels,the lowest color values in each color ramp are also often difficult tomeasure. Thus, in the preferred embodiment, the first point in each ramp(digital value 15) is discarded because it is typically inaccurate, andthe cardinality of S is adjusted down accordingly. In addition, in thecalculation of f(X,Y,Z;X_(K),Y_(K),Z_(K)), if X=X_(K), Y=Y_(K), andZ=Z_(K), the calculation is skipped and the cardinality of S is adjusteddown accordingly.

In the preferred embodiment, the above-described objective function is asum of squares of 96 differentiable functions in X_(K), Y_(K), Z_(K) (16points×2 xy-components×3 channels=96). Note that 16 points are usedinstead of the 17 that are measured in each ramp, since the lowest valueis discarded. Since the objective function is a sum of squares of 96differentiable functions, it is amenable to standard nonlinear leastsquares techniques such as the Levenberg-Marquardt algorithm.

As with any iterative algorithm for nonlinear least squares problems,the Levenberg-Marquardt algorithm requires an initial guess for (X_(K),Y_(K), Z_(K)). There are two candidates for the first guess. One is (0,0, 0) and the other is the actual measured black point of the displaydevice. Preferably, the actual measured black point is first used as theinitial guess. If a maximum of 100 iterations is exceeded withoutachieving a predetermined threshold value, then another round ofiterations is performed with (0, 0, 0) as the initial guess. Thethreshold value is the average distance of each point from its center ofmass, and in the preferred embodiment is set at 0.001. If the iterationusing the measured black point as the initial guess achieves thethreshold value, then the result is used as the estimated black point.If not, the result of the iteration using (0,0,0) as the initial guessis compared to the result of the iteration using the measured blackpoint as the initial guess, and the iteration result that gives thesmallest value for the object function Φ is used as the estimated blackpoint.

Turning back to FIG. 3, once the black point is obtained is step S302,the black point is subtracted from the raw XYZ color ramp measurementsin step S303. FIG. 4 depicts a graph showing black-point correctedmeasured values in the x-y chromaticity plane. Cluster 401 representsthe black-point corrected chromaticities of the green ramp, cluster 402represents the black-point corrected chromaticities chromaticities ofthe red ramp, and cluster 403 represents the black-point correctedchromaticities of the blue ramp. Points 404, 405 and 406 represent theblack-pointed corrected chromaticities of lowest value (digital value of15) of each color ramp. As can be seen in FIG. 4, points 404, 405 and406 lie far outside their respective clusters. This is due to theinaccuracy of the color measuring device at low light levels.

Next, the tristimulus matrix M is populated. Instead of populating thematrix with X, Y, and Z values corresponding to measured XYZ values whenthe maximum RGB values are applied to the device, the present inventionpopulates the tristimulus matrix with averaged, black-point correctedchromaticity values. In this way, the resulting tristimulus matrix notonly accounts for effects of flare, but also provides a more robustcharacterization that is less susceptible to inaccurate colormeasurements.

Initially, in step S304, the black-point corrected chromaticity valuesassociated with lowest value (digital value of 15) in each color ramp isdiscarded. For example, points 404, 405, and 406 in FIG. 4 would bediscarded. In addition, if the black point was estimated, any otherpoints in each color ramp that was not used to estimate the black pointshould not be used. Next, in step S304, the chromaticities for eachmeasure color cluster are averaged. The averaged chromaticity points arerepresented as follows:({overscore (x)}_(r), {overscore (y)}_(r)), ({overscore (x)}_(g),{overscore (y)}_(g)), ({overscore (x)}_(b), {overscore (y)}_(b))

Next, in step S305, scaling values for the averaged chromaticities arecalculated by solving the following equation for t_(r), t_(g), andt_(b). ${\begin{pmatrix}{\overset{\_}{x}}_{r} & {\overset{\_}{x}}_{g} & {\overset{\_}{x}}_{b} \\{\overset{\_}{y}}_{r} & {\overset{\_}{y}}_{g} & {\overset{\_}{y}}_{b} \\{1 - {\overset{\_}{x}}_{r\quad} - {\overset{\_}{y}}_{r}} & {1 - {\overset{\_}{x}}_{g} - {\overset{\_}{y}}_{g}} & {1 - {\overset{\_}{x}}_{b} - {\overset{\_}{y}}_{b}}\end{pmatrix}\begin{pmatrix}t_{r} \\t_{g} \\t_{b}\end{pmatrix}} = \begin{pmatrix}X_{W} \\Y_{W} \\Z_{W}\end{pmatrix}$X_(W), Y_(W), Z_(W) is the measured white point of the display device.The tristimulus matrix M is then populated in step S306 according to thefollowing equation. $\quad\begin{pmatrix}{t_{r}{\overset{\_}{x}}_{r}} & {t_{g}{\overset{\_}{x}}_{g}} & {t_{b}{\overset{\_}{x}}_{b}} \\{t_{r}{\overset{\_}{y}}_{r}} & {t_{g}{\overset{\_}{y}}_{g}} & {t_{b}{\overset{\_}{y}}_{b}} \\{t_{r}\left( {1 - {\overset{\_}{x}}_{r} - {\overset{\_}{y}}_{r}} \right)} & {t_{g}\left( {1 - {\overset{\_}{x}}_{g} - {\overset{\_}{y}}_{g}} \right)} & {t_{b}\left( {1 - {\overset{\_}{x}}_{b} - {\overset{\_}{y}}_{b}} \right)}\end{pmatrix}$

Once the tristimulus matrix is populated, gamma curves can be determinedusing the standard approach. The inverse matrix of tristimulus matrix isapplied to the measured XYZ data of the neutral ramp to obtain linearRGB data. Next, in the case of a CRT display, the linear RGB data iscorrelated with the digital RGB values using nonlinear regression on thegain-offset-gamma (GOG) model. In the case of an LCD display, the linearRGB data is correlated with the digital RGB values using direct linearinterpolation.

The invention has been described above with respect to particularillustrative embodiments. It is understood that the invention is notlimited to the above-described embodiments and that various changes andmodifications may be made by those skilled in the relevant art withoutdeparting from the spirit and scope of the invention.

1. A method for generating a color model that characterizes a displaydevice, the method comprising the steps of: generating a plurality ofcolors on the display device; measuring the generated colors;determining a black point and a white point from the measured colors;correcting the measured color values with the obtained black point inorder to obtain a plurality of chromaticity values; averaging thechromaticity values of the corrected color values; and generating atristimulus matrix with the averaged chromaticity values and thedetermined white point;
 2. The method according to claim 1, wherein theplurality of colors are generated by the display device in accordancewith a plurality of digital values in a red, blue, green and neutralramp.
 3. The method according to claim 2, further comprising the stepof: creating a plurality of gamma curves with the generated tristimulusmatrix and the measured colors, wherein the determining step includesthe steps of: inverting the tristimulus matrix, and multiplying theinverted tristimulus matrix with the measured colors from the neutralramp.
 4. The method according to claim 3, wherein the averaging stepdiscards the chromaticity value from the first corrected measured colorvalue of each ramp.
 5. The method according to claim 3, wherein thedisplay device is a CRT display device, and wherein the red, blue, andgreen ramps contain the digital values of 15 to 255 in steps of 15, andthe neutral ramp contains the digital values of 0, 8, 16, 32, 64, 128,192 and
 255. 6. The method according to claim 3, wherein the displaydevice is a LCD device, and wherein the red, blue, and green rampscontain the digital values of 15 to 255 in steps of 15, and the neutralramp contains the digital values of 0 to 255 in steps of
 15. 7. Themethod according to claim 3, wherein the plurality of colors aremeasured in XYZ.
 8. The method according to claim 7, wherein the blackpoint is determined by measuring the color produced at the neutral rampvalue of R=G=B=0.
 9. The method according to claim 7, wherein the blackpoint is determined by estimating the black point from the plurality ofmeasured colors using a non-linear optimization method.
 10. The methodaccording to claim 9, wherein the non-linear optimization method is theLevenberg-Marquardt algorithm.
 11. A color management module forconverting device-independent color values to device-dependent colorvalues, wherein the color management module utilizes a tristimulusmatrix generated by the method according to any of claims 1 to 10, toconvert device-independent color values into RGB values used by thecharacterized display device.
 12. An apparatus for generating a colormodel that characterizes a display device, the apparatus comprising: aprogram memory for storing process steps executable to perform a methodaccording to any of claims 1 to 10; and a processor for executing theprocess steps stored in said program memory.
 13. Computer-executableprocess steps stored on a computer readable medium, saidcomputer-executable process steps for generating a color model thatcharacterizes a display device, said computer-executable process stepscomprising process steps executable to perform a method according to anyof claims 1 to
 10. 14. A computer-readable medium which storescomputer-executable process steps, the computer-executable process stepsfor generating a color model that characterizes a display device, saidcomputer-executable process steps comprising process steps executable toperform a method according to any of claims 1 to
 10. 15. A method forgenerating a color model that characterizes a display device, the methodcomprising the steps of: obtaining measurement data of plural colorsdisplayed on the display device; estimating a black point of the displaydevice from the measurement data; and generating the color model thatcharacterizes the display device based on the estimated black point andthe measurement data.